Cerebral ischemia refers to cessation or reduction of blood flow to the cerebral tissues. Cerebral ischmia can be characterized as either global or hemispherical. Hemispherical or focal ischemia refers to cessation or reduction of blood flow within the cerebral vasculature resulting from a partial or complete occlusion in the intracranial or extracranial cerebral arteries. Global ischemia refers to cessation or reduction of blood flow within the cerebral vasculature resulting from systemic circulatory failure from various causes, including aortic surgery, cardiac arrest, shock, and trauma. Circulatory arrest is often required in performing surgeries on the aorta, e.g., aneurysm repair, aortic dissection, endarterectomy of aortic atheroma, and aortic stenting. Blood flow through the aorta is often interrupted due to opening of the aorta for these surgical procedures. Cessation of systemic circulation therefore places a patient at great risk, particularly in the cerebral vasculature where ischemia can rapidly lead to irreversible neurologic damage.
Various techniques have been proposed to improve cerebral perfusion in a patient suffering from either global or focal ischemia. For example, conventional cardiopulmonary bypass is used during cardiovascular surgeries to maintain perfusion to peripheral organs during cardiac arrest. However, cardiopulmonary bypass is generally not useful when the aorta fails to remain intact during aortic surgeries. Retrograde aortic perfusion (RAP) has been proposed to improve cerebral perfusion by clamping the ascending aorta and perfusing the aorta in a retrograde direction through a peripheral arterial access, typically the femoral artery. Disadvantages associated with retrograde aortic perfusion include significant cerebral embolization from dislodgment of atheromatous material in the descending aorta and the aortic arch. Moreover, RAP is not useful for aortic procedures distal or proximal to a limited surgical region of the aorta.
Another technique for protecting the brain during global or focal ischemia is provided by hypothermic circulatory arrest (HCA). HCA is achieved by inducing marked systemic hypothermia prior to cessation or reduction of systemic circulation. There are several disadvantages associated the HCA. For example, during cardiac arrest from cardiac arrhythmia or aortic surgeries, the systemic circulation remains stopped, thereby placing the patient at significant risk of ischemia despite utilizing hypothermia. Moreover, HCA has been associated with systemic coagulopathy, typically disseminated intravascular coagulopathy. Therefore, aortic surgery performed with HCA is associated with relatively high mortality (approximately 20 percent).
Another proposed technique for cerebral perfusion is referred to as selective antegrade cerebral perfusion (SCP). SCP is achieved by introducing a catheter through the aorta into a carotid artery to perfuse the cerebral vasculature. However, cerebral embolization can occur from introduction of the catheter which can dislodge atheromatous material, often present at the take-off from the aorta. Cerebral embolization can also occur from dislodgment of atheromatous material by clamping or snaring of the carotid artery. Air embolization can also occur from insertion of the catheter in the aorta or the carotid artery.
Another technique for improving cerebral perfusion is provided by retrograde cerebral venous perfusion (RCP), which is achieved by clamping the inferior vena cava and introducing oxygenated blood through a catheter inserted in the superior vena cava. Flow is established in a retrograde direction up the vena cava into the brachial and jugular veins. However, there are several disadvantages associated with using the RCP. For example, a majority (approximately 80%) of the oxygenated blood will run off into the arms and/or the heart and lungs, with as little as 20% of the blood entering the brain. Secondly, valves in the jugular veins may obstruct blood flow into the intracranial venous system. The blood can flow outwardly through the extensive collateral circulation without perfusing the brain. The amount of blood returned to the aorta from the carotid arteries represents no more than approximately 5% of the blood initially introduced to the superior vena cava. Thirdly, such retrograde perfusion often results in an increase in cerebral pressure which further inhibits blood inflow. Arterial efflux to the cerebral vasculature is found by the inventor to disappear after 20 minutes. Therefore, during RCP, the venous flow rates and pressures required to achieve and maintain significant arterial efflux are highly variable.
For these reasons, it would be desirable to provide improved devices and methods for protecting the brain and cerebral vasculature of patients suffering from global or focal ischemia, without the risk of cerebral embolization or systemic side effects.